This invention relates to a new design for lasers. A major application is a low-cost driver for inertial confinement fusion (ICF) as an energy source, and more particularly, for providing the required very large laser energy in a short optical pulse at low cost.
Over the past approximately 45 years, a substantial effort has been generally directed towards obtaining thermonuclear fusion energy from targets irradiated by various types of particle beams. Lasers, particularly those with short wavelength light (0.25-0.5 microns), have generally emerged as the main contender for producing ignition (substantially more fusion energy out than the energizing beam puts in). The glass laser, where the laser energy is stored in a solid doped crystalline or glass medium and then extracted in a short pulse, has generally been the primary type of laser used in investigations to date (see, for an example of a general description, E. Snitzer, “Glass Lasers,” Applied Optics, Vol. 5, No. 10, pp. 1487-1499, 1966, incorporated by reference herein for all purposes). The United States has generally supported the construction and operation of a National Ignition Facility (NIF). This laser uses Nd:glass (discovered in 1961) for the storage medium and should produce about 2 megajoules of laser light at a wavelength of 0.35 microns that is ⅓ the wavelength of the fundamental Nd frequency in a pulse length of approximately 10−8 seconds (see, for example, “The National Ignition Facility: Laser System, Beam Line Design and Construction,” by R. H. Sawicki, in M. A. Lane and C. R. Wuest (Eds.), Optical Engineering at the Lawrence Livermore National Laboratory II: The National Ignition Facility, Proceedings of SPIE, Vol. 5341, 2004, pp. 43-53, incorporated by reference herein for all purposes). The $4 to $5 Billion Dollar cost of the facility is generally leading to a cost per joule of laser energy on the order or in excess of $2,000/joule. The efficiency and repetition rate may not be suitable for commercialization. In addition, the provision of such energies is limited in pulse length for pulses in the 10−9 second range, although a shorter pulse length may be desirable for some targets. For a summary of thermonuclear fusion efforts (see, for example, M. Moyer, “Fusion's False Dawn,” Scientific American, March, pp. 50-57, 2010, incorporated by reference herein for all purposes). Thus, improved lasers for ICF would be beneficial, both in terms of speeding up the development cycle and eventual commercialization.
An alternative approach to such short pulse lasers may be to use long laser pulses that are then time compressed. Many people have generally discussed overall architectures for pulse compression of efficiently generated long laser pulses, particularly those using the krypton fluoride laser (discovered in 1975) with various compression techniques. Pure angular multiplexing received substantial attention, in part due to its conceptual simplicity and potential for a factor on the order of 10 in cost reduction compared to the glass laser technology (see, for example, R. O. Hunter, Jr., and D. L. Fried, “High Energy Laser,” U.S. Pat. No. 4,337,437, Jan. 29, 1982; R. O. Hunter, Jr., “Compressed Pulse Laser,” U.S. Pat. No. 4,264,869, Apr. 28, 1981; R. O. Hunter, Jr., et al., “Key Technical Issues Associated With A Method Of Pulse Compression,” U.S. Department of Energy, DOE/DP/40107-1, 1980, “Excimer Lasers for ICF,” by L. A. Rosocha, S. J. Dzuchlewski, B. J. Krohn and J. McLeod, in Nuclear Fusion by Inertial Confinement: A Comprehensive Treatise, by G. Velarde, Y. Ronen and J. M. Martinez-Val (Eds), CRC Press, Inc., 1993, Chapter 15, pp. 371-420, incorporated by reference herein for all purposes).
In addition to, or in conjunction with, angular multiplexing, the uses of stimulated scattering processes for proposed large-scale applications, particularly Raman and Brillouin scattering for pulse compression, were generally developed later than the glass laser technology for ICF. In various combinations with pure multiplexing, for example, they were shown to time compress krypton fluoride light for the ICF application at small scale (see, for example, M. J. Shaw, J. P. Partanen, Y. Owadano, I. N. Ross, E. Hodgson, C. B. Edwards and F. O'Neill, “High-Power Forward Raman Amplifiers Employing Low-Pressure Gases in Light Guides: II. Experiments,” Journal of the Optical Society of America B, Vol. 3, No. 10, pp. 1466-1475, 1986, incorporated by reference herein for all purposes).
In addition, Raman scattering was generally demonstrated as a technique to combine separate apertures to provide near diffraction-limited output at large scale for non-ICF applications (see, for example, A. Hunter and G. Houghton, “Single Pulse Excimer Ground Based Laser ASAT Concept Definition Study,” Thermo Electron Technologies Corporation, TTC-1588-R, 1989, incorporated by reference herein for all purposes; and N. Bloembergen et al., “Report to The American Physical Society of the Study Group on Science and Technology of Directed Energy Weapons,” Reviews of Modern Physics, Vol. 59, No. 3, Part II, pp. S1-S201, 1987, incorporated by reference herein for all purposes). In previous architectures for ICF, the stimulated scattering was generally not used to replace reflective elements or to provide reflectivities at very high fluence, only to time compress pulses in geometries where material mirrors controlled the input and output from the scattering region(s). Thus, the architectures were generally limited by the properties of the optical elements in terms of handling the inputs and outputs for the compression regions (see, for example, J. J. Ewing et al., “Optical Pulse Compressor Systems for Laser Fusion,” IEEE Journal of Quantum Electronics, Vol. QE-15, No. 5, pp. 368-379, 1979, incorporated by reference herein for all purposes, and M. J. Damzen and H. Hutchinson, “High-efficiency Laser Pulse Compression by Stimulated Brillouin Scattering,” Optics Letters, Vol. 8, No. 6, pp. 313-315, 1983, incorporated by reference herein for all purposes). Since the cost of large-scale systems may depend markedly on the overall optical area, the higher the operating fluences on the optical surfaces and the lower the number of optical surfaces, the lower the cost of the compression step. In addition, the material window regions between the vacuum of the target region and the laser region may pose limitations due to their damage properties.
In some cases, the solid state lasers for ICF may have been strongly limited by the damage fluence in the glass storage media itself as well as in the beam handling optics. In addition to the damage fluence, other parameters, such as the nonlinear index of refraction, may be superior for certain embodiments described herein from a laser design standpoint. Also, some preferred architectures for ICF lasers based on such storage media entail generating a short pulse of light of the desired optical pulse length (≅10−8-10−9 seconds) at an infrared wavelength (1.06 microns) and then utilizing a frequency tripling technique in solid materials to convert the laser light to an ultraviolet wavelength (0.35 microns) more desirable for target coupling. NIF utilizes such an architecture. The conversion elements may be subject to damage fluence limitations as well. Characteristically, the damage fluences in the ultraviolet are on the order of 1-10 joules/cm2 for such ultraviolet light with a pulse length of about 10−8 seconds.
In a previous assessment of designs associated with nonlinear scattering techniques for pulse compression as applied to ICF, the following quotes were given:
1) “Nonlinear schemes have the advantages of allowing for beam cleanup, accommodating a combination of pump beams, and reducing the requirements on the optical beam quality of the pump pulse. However, Raman compression suffers from inefficient conversion of the pump pulse into the compressed pulse and limited power and intensity gains. Typically, the compression ratio for backward Raman is limited to less than 5 at 50% conversion efficiency, due to parasitic depletion of the medium by the second Stokes-shifted pulse. The efficiency can be increased, but the compression ratio must be lowered in the process. Therefore, to use Raman compression efficiently, it must be combined with multiplexing to get a high compression ratio. For short pulses, Brillouin compression does not exhibit the limitations of Raman compression for power gain and efficiency. Typical experimental results show efficiencies of 40 to 80% and compression ratios of 2 to 80; however, the bandwidth is even narrower than it is with Raman compression. Therefore, SBS compression for KrF laser beams may be more promising in reducing the number of beamlines, although the broad bandwidth advantages are not retained. However, modeling of SBS converters have shown this process to be incompatible with pump duration longer than ˜50 nsec, making it unsuitable as a substitute for angular multiplexing.
Hybrid schemes, involving combinations of multiplexing and Raman or Brillouin compression, have been invented in attempts to design scalable systems which incorporate the best features of both multiplexing and nonlinear compression. However, so far these schemes have not proven to be simpler, more efficient, more economical, or more readily scalable than multiplexing” (L. A. Rosocha, supra, p. 2).
2) “In view of the recommendations made last year in the report of the DOE KrF Panel, it appears that some elaboration of our reasons for rejection of nonlinear optical pulse compression techniques in favor of optical angular multiplexing is appropriate here. This decision is largely based on the conclusion that for large systems, the size of individual optics downstream from the final amplifiers is already sufficiently large that using nonlinear optics to compress the energy in larger time slots or to combine the outputs of several amplifiers does not result in any cost savings. Furthermore, there are efficiency losses in any nonlinear conversion process, and the process requires additional optical elements that increase the cost. Finally, the nonlinear techniques impose severe constraints on the bandwidth that can be compressed and thus do not provide the bandwidth flexibility that is believed to be desirable for an ICF facility. Although detailed designs have not been done in the context of the LMF, it appears that the following considerations make nonlinear pulse compression techniques noncompetitive with angular multiplexing. These considerations were discussed at the KrF Workshop held in Santa Fe in April 1989. There appeared to be general agreement among representatives all of the major laboratories that have addressed these issues in KrF that, although nonlinear compression techniques might present some cost savings in relatively small, short-pulse systems that would otherwise require a large number of beamlets, there were no obvious cost savings in large systems and no workable methods for achieving large bandwidths, aside from the possibility of generating a comb of multiple narrow frequencies in the front end to drive different amplifiers and Raman cells. We believe this adds unnecessary complexity and may not have as beneficial effect as a truly broadband source.” (N. A. Kurnit, “Nonlinear Pulse Compression,” Inertial Confinement Fusion at Los Alamos, Vol. 1, Ch. VII, pp. 1-2, 1989, incorporated by reference herein for all purposes.)